The present invention relates to a microwave excited electrodeless light source and, more specifically, to a solid state microwave power source for use in the light source.
In a microwave powered electrodeless light source of the type described in the U.S. Pat. No. 3,943,403, it is necessary to provide sufficient microwave power to the electrodeless lamp for both starting the lamp and just after glow initiation so that the lamp goes through the transition period from low pressure glow to higher pressure arc quickly and safely with respect to the solid state power device and reaches the full running condition. The load impedance presented to the microwave power source just after glow initiation is quite different than that presented when the lamp is in the running condition. In the glow condition, the lamp impedance has a very high real part, of the order of 1500 ohms. In the running condition, the lamp impedance is lower, of the order of 150 ohms, and has been effectively matched to the output impedance of the power source via the lamp fixture and its impedance matching schemes.
At microwave frequencies the load during the glow condition results in a very high voltage standing wave ratio (VSWR) on the transmission line between the source of microwave power and the load. High voltage standing wave ratios may cause excessive voltages at the collector of the amplifier transistor in a microwave power source, and this condition could cause breakdown. In addition, since a high VSWR type load results in reflection of forward directed power back into the microwave power source, the source must dissipate this power as heat. This may result in a thermal overload. The full running condition has a finite impedance determined by lamp parameters, such as lamp geometry, chemical fill material, operating frequency, wall temperature and pressure. Since the performance of any unregulated power source is load dependent, and in particular a solid state microwave power source is extremely load dependent, it is necessary to promote the compatability of the solid state microwave power source with the input impedance of the fixture housing the lamp by careful design of the power source.
A component called a circulator inserted in the line between the microwave power source and the fixture for the lamp has been used previously to protect this microwave power source. This device contains a magnetoactive ferrite material which has non-reciprocal properties such that it directs the flow of power. The circulator allows the maximum forward power to be delivered to the lamp, and in the case of an impedance mismatch, such as when the lamp is off or just after the glow is initiated, the reflected power is diverted by the circulator to a resistive termination and dissipated as heat. While this device does protect the power source, the device along with its associated terminating resistor is bulky and expensive and represents a power loss in the line. Its use is not desirable in a practical electrodeless lamp system.